Design complexity is here defined as the complexity a designer experiences when faced with a design situation. Almost all design situations offer potentially infinite and limitless sources of information, requirements, demands, wants and needs, limitations, and opportunities. These “infinite and limitless sources” usually present themselves in the form of diverse technological possibilities, numerous and constantly changing contextual factors and societal preconditions, sophisticated and/or non-informed clients, customers, and user demands and desires. Even though all of these sources can inform the designer about a potential design, it is not possible to exhaustively explore them for all potentially useful information. Facing such “infinite” information sources might lead a designer (even an experienced one) to experience an overwhelming design complexity. The designer has to make all kinds of decisions and judgments, such as, how to frame the situation, who to listen to, what to pay attention to, what to dismiss, and how to explore, extract, recognize, and chose useful information from all of these potential sources. An inexperienced designer might suffer from “design paralysis” when confronted with such endless opportunities. These design situations are sometimes characterized as “under determined” problems, or in Schön’s words as a “messy” situation, or in Rittel’s words a “wicked problem. According to the definition presented here, it is not possible to objectively measure design complexity. We cannot, based on some objective measurement, argue that “this situation has a higher design complexity than that situation.” Instead, design complexity is the designer’s subjective experience of complexity. This experience is a consequence of the nature of the task in the specific situation, in relation to the specific purpose, and in relation to the professional skill, competence, and experience of the designer. This means that one designer might experience a particular design situation as complex, while another might not.